1. Fill the plastic storage bag with water until the bag is about three-quarters full and then seal it.

2. Hold the bag of water at the top with one hand. With the other hand, push a well-sharpened pencil through the bag so that the pencil is sticking out on both sides. See how many pencils you can add until the bag leaks.

What happened? Why didn't the bag leak when pierced with a pencil?

Plastic bags are made up of polymers, or chains of molecules, that bend and stretch. When you poked pencils through the bag, the polymer strands shifted aside to allow the pencils through. They quickly re-formed around the pencils, keeping water sealed inside.